The supply of hydrocarbon fuels is limited. When hydrocarbons are used as energy carriers and converted into mechanical or electrical energy they generate polluting by-products. The need to replace hydrocarbon based energy carriers with non-polluting renewable energy carriers is recognized worldwide.
Pure hydrogen has been identified as a potential renewable energy carrier. When electrical energy produced in one location from renewable sources, such as sunlight, wind, or hydroelectric power, is converted into hydrogen, and the hydrogen is subsequently converted at another location back into electrical energy in a fuel cell, hydrogen acts as an energy carrier and no pollution is inherently produced. However, the transportation and storage of hydrogen is difficult and volumetrically inefficient due to the small size of the hydrogen molecule and its volatility in air. Further when gaseous hydrogen is used, rare and expensive catalysts must be incorporated into the fuel cell in order to convert molecular hydrogen into ionic materials that can be transported across the fuel cell membrane. The inefficiency in this system is not a result of the energy conversion devices it is an inherent fault in the nature of the energy carrier. In other words the problem with this system of renewable energy is not the fuel cell it is the fuel.
It has long been known that other ionic materials such as alkali metal ions may be transported across membranes to combine with oxygen and produce electrical energy in a process analogous to hydrogen/oxygen fuel cells. Shuster, In U.S. Pat. No. 5,525,442 entitled “Alkali Metal Battery”, describes a battery with a solid alkali metal anode deposed in a non-aqueous medium and separated by a membrane from the oxygen cathode contained in water.
For the purposes of this disclosure the term battery refers to an energy conversion device where the quantity of at least one of the reactants is predetermined by the size of the conversion device and the total quantity of said reactant available for conversion is contained within the structure of the battery. In the case of the Shuster battery described above, the cell uses a solid lithium anode. The scale (size, energy capacity) of the battery is directly proportional to the scale (size, energy capacity) of the anode.
For the purposes of this disclosure, the term fuel cell refers to an energy conversion device where the quantities of both chemical reactants are supplied from a source external to the conversion device. Thus the scale of the conversion device may be independent of the quantity of the reactants. The quantity of both positive and negative chemical reactants can be adjusted or replenished without modifying the conversion device.
Alkali metals are attractive candidates for metal/air and metal/water batteries because of their inherently high energy densities. They have generally not been commercially acceptable, due at least in part, to difficulties in controlling parasitic corrosion reactions and the tendency of the materials to thermally “run away”. Fuel cell and battery art has long taught the limiting necessity of separators, spacers, membranes, porous barriers, dynamic films, mercury amalgams, alloys with less active metals, non-aqueous electrolytes, or high temperature molten salts between the cell electrodes to prevent mechanical shorting and prevent direct, violent chemical combinations where alkali metals were utilized. In recent years new materials called intercalation compounds or insertion compounds have been developed for alkali metal batteries, and in particular for lithium batteries, to mitigate these runaway conditions.
When hydrogen is used as the insertion material the resulting compounds are often referred to as reversible hydrides and the bond between the intercalated host material and the inserted hydrogen is called an occluded bond. For the purposes of this invention we restrict our definition of intercalation compounds to only those materials that can be intercalated with hydrogen, alkali metals, alkali metal hydrides, or combinations of hydrogen and alkali metals.
Intercalated materials may be conceptualized as compounds being comprised of two components, a “host” material, and a visiting insertion material or “intercalate”. The host material may be defined as elements, naturally occurring intermetallic compounds, or synthetic compounds and structures that allow the reversible insertion of ions, atoms, or molecules of another material—the insertion material or intercalate—within spaces in the host structure. The bonding of the host material with the intercalate does not adversely change the chemical-to-electrical energy conversion properties of the intercalate significantly. For the purposes of this disclosure, reversible intercalation may be defined as a property of a host intercalation material to repeatedly accept the insertion and removal of an intercalate. Methods for the insertion of the intercalate are numerous and well known, including electrical, chemical, and mechanical methods. Said intercalation methods do not form a part of the present invention and any appropriate method for producing an intercalated material may be employed.
It is well known in the art that metal hydrides and certain nanostructured materials such as graphitic carbon, carbon nanotubes, house-of-card (HOC) structure MoS2, alkali metal/carbon structures, layered silicon structures, and many others can be made to reversible intercalate or occlude hydrogen. These materials can also be fractured by known methods into particles with dimensions suitable for classification, when dispersed in liquids, as sols. Although these dispersed materials may be used directly in fuel cells, and their use without alkali metals would not depart from the scope of this invention disclosure, on the basis of energy density these materials are typically less efficient than lithium intercalation compounds. It would be beneficial with respect to energy density if the intercalation host and/or the alkali metal that is intercalated, as described above, could also be intercalated with hydrogen.
Further, hydrogen may be intercalated in two forms. Most often, in metal hydrides that are unsuitable for battery alloys, like magnesium hydride, calcium hydride, and AB alloy hydrides, etc., the hydrogen is retained as molecular hydrogen (H2). However, hydrogen intercalated in nickel metal hydride batteries or in other nanostructured materials such as exfoliated transition metal dichalcogenides as described in U.S. Pat. No. 4,229,196 to Woollam, entitled “Atomic Hydrogen Storage Method and Apparatus”, is retained as atomic hydrogen. In hydrogen/oxygen fuel cells, noble metal catalyst are required to “break” molecular hydrogen into atomic hydrogen prior to use in the cell. This requirement adds significant cost and complexity to the fuel cell. It would be beneficial if the intercalation host and/or the intercalated alkali metal could be induced to bind atomic hydrogen, as for example in LiH, and thereby mitigate, reduce, or eliminate the need for noble metal catalyst in fuel cells.
Alkali metal intercalation compounds have recently been commercialized for use in lithium ion batteries. These compounds help to limit the quantity of free lithium metal in the cell. They are often employed as cathodes that can accept a lithium ion on discharge and hold it safely until it is re-plated on the lithium anode during recharge. However, these materials may also act as anodes in some configurations such as the “rocking chair” battery where lithium ions are transferred back and forth between two carbon electrodes. It has only been possible to commercialize lithium batteries for consumer products because these new intercalation materials can reduce the free lithium in the cells and mitigate the parasitic corrosion reaction with its attendant release of heat. Lithium ions can store significantly more energy on a volumetric basis than hydrogen in a practical manner. Lithium ions can also be induced to travel through membranes to react with oxygen and in the process generate an electric current.
Finally, many alkali metal intercalation compounds can be formed or fractured into small particles by known methods. Once fractured, they can remain suspended in liquid electrolytes for extended periods of time. In fact, many of these alkali metal intercalation compounds may be easily broken or formed into particles that when dispersed in liquids fit within the dimensional definition of a sol. Many of them will remain dispersed in liquid electrolytes for months. Some of them can be formed into particles that will remain suspended for years. It would be beneficial for volumetric energy efficiency and many other benefits if these materials could be directly circulated as anodes in fuel cells.
As described above, alkali metal/air batteries are well known in the art. Controlling the parasitic corrosion reaction is a critical barrier for their commercial acceptance. Many methods have been described in the literature for achieving this control. For example, Rowley in U.S. Pat. No. 3,791,871 entitled “Electrochemical Cell” describes a method that utilizes the reactant by-product, in this case lithium hydroxide, to limit the reaction.
Many have found ways to control the corrosion reaction other than modifying the chemical composition of the anodes, cathodes, separators, and electrolytes. For example, Littauer et al. in U.S. Pat. No. 4,481,266, describes a method for controlling the quantity of electrolyte so that only a portion of the cathode is exposed in the reaction at any given time. However, all of the methods disclosed to date, that rely on alkali metals for anodes, have overlooked the self-limiting benefits and flow characteristics afforded by intercalation compounds. Further, none of them have described the additional benefits in energy density and cost that can be achieved with particles comprised of intercalated alkali metals, alkali metal hydrides, or occluded hydrogen. It would be beneficial if a high energy density material with flow characteristics and viscosity that allowed the duration of the reaction and quantity of reactants to be controlled by the flow of the anode material could be employed. It would also be beneficial if the transport of the reactant allowed the use of existing liquid handling systems currently employed to handle liquid hydrocarbons.
In addition to flow characteristics and viscosity, controlling the particle size of the intercalation host may enhance other benefits. For example the additional benefits in rate of heat transfer and particle suspension time that may be enjoyed by employing nanodimensional particles of materials are well known. Enhancing the heat transfer of the liquid fuel of the present invention would provide an additional means to mitigate the deleterious effects of parasitic corrosion. Therefore, it would be beneficial if the intercalation compound could be comprised in part of particles where at least one dimension of the intercalation host was in the sub-micron or nanodimensional range.